X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Mon, 12 Jul 2010 09:41:14 -0400 Message-ID: X-Original-Return-Path: Received: from imr-db03.mx.aol.com ([205.188.91.97] verified) by logan.com (CommuniGate Pro SMTP 5.3.8) with ESMTP id 4388384 for lml@lancaironline.net; Sat, 10 Jul 2010 23:21:04 -0400 Received-SPF: pass receiver=logan.com; client-ip=205.188.91.97; envelope-from=Sky2high@aol.com Received: from imo-da03.mx.aol.com (imo-da03.mx.aol.com [205.188.169.201]) by imr-db03.mx.aol.com (8.14.1/8.14.1) with ESMTP id o6B3KPW0001773 for ; Sat, 10 Jul 2010 23:20:25 -0400 Received: from Sky2high@aol.com by imo-da03.mx.aol.com (mail_out_v42.9.) id q.cfd.78b83f97 (43900) for ; Sat, 10 Jul 2010 23:20:22 -0400 (EDT) Received: from magic-m22.mail.aol.com (magic-m22.mail.aol.com [172.20.22.195]) by cia-dc06.mx.aol.com (v129.4) with ESMTP id MAILCIADC065-ab7c4c3938762a9; Sat, 10 Jul 2010 23:20:22 -0400 From: Sky2high@aol.com X-Original-Message-ID: <31a02.166fdad6.396a9276@aol.com> X-Original-Date: Sat, 10 Jul 2010 23:20:22 EDT Subject: Re: [LML] Small tail, MK II tail, CG range X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="part1_31a02.166fdad6.396a9276_boundary" X-Mailer: AOL 9.5 sub 155 X-AOL-ORIG-IP: 67.175.87.113 X-AOL-IP: 172.20.22.195 X-Spam-Flag:NO X-AOL-SENDER: Sky2high@aol.com --part1_31a02.166fdad6.396a9276_boundary Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Wolfgang, Nice analysis. However, I disagree with your conclusion ",,, live with the limitations or install a larger tail to give that extra pitch authority." The flap is as much a flight control tool as the elevator and, as you stated, affects the CG end points. Should landings be difficult because there is insufficient elevator authority for holding the nose up, reduce the flap setting from 45 down (300 series) to something less. Note that 200 series aircraft do not provide for such a large flap deployment with their itsy bitsy tail. Some 300 fliers prefer no more than flaps 30 on landing. On the other hand, if rear CG is a concern in cruise flight, ticking the flap a bit out of reflex may help resolve this when in such an uncomfortable flight regime. A slightly slower speed must be endured and if the flap is taken completely out of reflex (normal wing), the IAS should probably be no greater than Va because of the stress on the wing/flap combo. Note that Lancair's chief pilot recommended that the 200 series flaps be fully reflexed above 140 KIAS (or was it 160?). All 300 series with standard landing gear should be using the Lancair recommended 5/16" (?, >1/4") final push rods since the 1/4" ones were known to be capable of being bent. Outback gear uses a different deployment technique. I am not sure a bigger tail helps when one approaches the "neutral point." I once flew in a rear CG heavy condition where even the autopilot couldn't quite hold level flight - instead a mild sinusoidal line was flown until 60 pounds of fuel was burned off. Too bad I didn't try to remove some reflex from the flaps - I was younger then. It would be interesting to know if a test pilot did stalls or other unusual attitudes with the flaps in different positions. Since some LNC2s have fallen out of the sky at slow speeds and close to the ground, perhaps it would have been useful to know the flap position. Because of the critical positioning of the partial rib where the final flap bell cranks are attached, the rate/angle of each flap could be different as it is moved through its range. Indeed, flap rigging has often been used to eliminate a heavy wing at cruise - ergo, deployment of each could be a bit different. It will be interesting if you are able to compute or formulate the computation for the CG range limits depending on the wing shape. Scott Krueger 320, small tail, std gear, gap seals In a message dated 7/10/2010 8:28:50 P.M. Central Daylight Time, Wolfgang@MiCom.net writes: The LNC2 uses the NLF(1)-0215F airfoil. A lot can be found by doing a Google search on that number. More detail can be found by going to Google for "NASA Technical Paper 1865". I have not taken the time to reverse engineer the CG range of the LNC2 but let me offer some observations. The airfoil used has long been touted as "the greatest thing since sliced bread" for General Aviation and it definitely has some advantages. But it's not new. Compare this airfoil to the P-51 airfoil and you will see some close similarities. The LNC2 being composite construction instead of aluminum lets the airfoil show more of it's theoretical advantages. It's a laminar shape with a good drag bucket. That bucket can be made to move to the lower Cl (lift coefficient) ranges with reflex allowing noticeably lower drag at higher cruise speeds. Along with reflex, the Cm (moment coefficient) goes positive, the center of lift of the wing travels forward giving a nose up force requiring down trim. This is in addition to the usual nose up force that goes with most all airfoils at high speed before considering flaps. With down flap, the drag bucket will move to higher Cl's making slower flight more efficient. And, of course, the Cm goes negative giving a nose down force requiring up trim. . . . and appropriate variations in-between . . . So, the rear CG limit is determined by high speed flight and available control authority, and the forward CG is determined by low speed / landing flight and available control authority. What is becoming clear here is that the center of lift does quite a bit of traveling fore and aft which is exaggerated by allowing negative or "cruise" flaps. Since you can't shift the CG during flight, you need a large amount of pitch authority from the tail to cover that range of lift travel. You have two choices in the LNC2, live with the limitations or install a larger tail to give that extra pitch authority. . . . A larger tail area can also help with abnormal attitude recovery. Wolfgang ____________________________________ From: Terrence O'Neill Subject: Re: [LML] LNC2 CG range Query Date: Wed, 07 Jul 2010 09:32:19 -0400 To: lml@lancaironline.net Dear humble sevant... or is it 'savant'? See, we often shackle our efforts to understand by having aforehand made up conventions to better understand something else. I feel motivared to offer some observations on understanding the L2s, if I may. Don't forget that you have a delete button if this gets too boring. This is how I see the Lancair's wing-flap-stability-stall, as compared to recent LML discussions of same. The CG range and the wing's characteristics is a good example of thinking 'in the box'.. Guys working in the wind tunnels measuring forces of invisible air found that at those small angles where wings make a lot more z force than Y force -- those forces seemed to be centered around 25% chord... which they first called a center of pressure, and then 'aerodynamic center' or a.c., and so they made their charts about this imaginary point and things work out great for calculations. The thing is, what's really going on is that as the moving wing flows through the stationary air, a pressure bubble is generated, with most of it being at the leading edge, and then tapering 'triangular-sish-ly' toward the trailing edge.... at least at higher angles of attack. And the lower or cruise angle so attack the bubble flattens out aft-ward... and is varied by moving a trailing edge flap. So this is getting at the importance of the bubble... that's what we move around when we move the pitch control. With a flap, or an aileron, or a tail flap thing. And so, when we talk about flying with the CG range forward or aft, we're really talking about the CG moving with respect to this total-airplane-bubble... which we have to keep centered over our center of mass. We do this by making little bubbles of pressure on the tail, or in Lancair reflexing of the flap, by moving the bubble of the wing. And that's what the CG or center of mass is hanging from. So when we drop a flap, it's obvious that we have moved that big bubble, and have to balance it with changing the bubble size on the horizontal tail.,, trim tabbing it. The design characteristics of the L@'s reflexible airfoil are referenced to the section with the flap not reflexed. Also, therefore, it is most likely that Lance figured the CG that way, because that's the way the aerodynamic data was available to him in the NASA report, I think, on the NLF(1)-0215F... please hasten to correct me if I'm wrong. : ) I just try to keep my (total airplane) bubble's center as close to the CG, wherever that is, by making a little bubble over or under the horizontal tail, with the pitch trim tab. One problem is that we pilots don;t talk much abut the shape or location of the pressure bubble at AOAs higher than the stall angle... and that's a situation where designers then have to start gluing yukky shapes of strakes and vanes on to correct this oversight. What Lance should have done in the first place is wind-tunnel his 200 and -235 from zero AOA up to 90 degrees, and he would have seen a big forward movement of the bubble's center resulting from the broad cowl and skinny aft fuselage... imo. Such testing was done by NASA for the Piper canard (after it crashed) and on a configuration like the Dragonfly... which I discovered when researching the Dragonfly we bought... and discussed in a Kitplanes article many years ago. In the too frequent event that a new design configuration is locked in by building before testing, the economical remedy solution then is to fly with a forward CG, or to add strakes aft, to keep that total-airplane high AOA bubble centered where it belongs ... aft of the design CG,.. for a restoring pitching moment In the Dragonfly we flew at forward CG. In L235/320 N211AL I have modified the horizontal tail to add slots to prevent it from stalling at AOA higher than the wing's stall AOA...( still testing that, but it worked on my Magnum.) Also I added ballast to the engine mounts to be certain the CG stayed forward, within Lance's original limits. It's a beautiful little plane, and very efficient ... and this is how it works -- I think. Terrence L235/320 N211AL On Jul 7, 2010, at 2:18 AM, _Sky2high@aol.com_ (mailto:Sky2high@aol.com) wrote: If you know or even care: In general, LNC2's as originally designed seemed to better tolerate a CG at the forward edge of the envelope rather than flight at or towards the rear. This includes adequate elevator control at flare during landing. Lancair tested the long engine mount on an LNC2 that moved the forward CG edge +1.5" and there were no flight problems. Hmmmmmm......... Consider that the LNC2 wing has a dramatic change in pitch forces when the flap is moved between its designed standard position and into -7 degrees reflex. In my airplane at around 140-160 KIAS the difference in those two flap positions is approximately a measured 6 degrees in attitude (couldn't measure the AOA delta). It is clear that moving the flaps sightly out of reflex (1 or 2 degrees) can help resolve uncomfortable flight at rear CG conditions by pitching the nose down some and altering the AOA. Perhaps the rear CG and small tail at cruise leads to some flight instability that cannot be overcome by the size of the tail? So, here is the question: If the CG range was calculated for the normal state of the wing (flaps not in reflex), is it possible that the range is too far back for normal cruise flight with the flaps in full reflex? If so, should the POH aircraft data include two ranges based on these two flap positions? What does such a change do to the forward CG limit? Of course, this might raise the same question with the 200 series aircraft. Why? Well, the faired in position of the flaps for 200 series aircraft is the not-in-reflex position while the plane cruises with the flaps reflexed and not faired in. The 300 series aircraft has the flaps in reflex when they are faired in to the fuselage. When considering an answer, remember that wings designed to operate by changing shape (TE goes through some reflex angles) have been primarily used in tailless airplanes and the TE position controls the pitch balance of the airplane. I have no idea how the CG range for such an aircraft is determined. Your humble servant, Grayhawk --part1_31a02.166fdad6.396a9276_boundary Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable
Wolfgang,
 
Nice analysis.  However, I disagree with your conclusion ",,, li= ve=20 with the limitations or install a larger tail to give that extra pitch=20 authority."
 
The flap is as much a flight control tool as the elevator and,= as you=20 stated, affects the CG end points.  Should landings be difficult beca= use=20 there is insufficient elevator authority for holding the nose up, reduce= the=20 flap setting from 45 down (300 series) to something less.  Note that= 200=20 series aircraft do not provide for such a large flap deployment with their= itsy=20 bitsy tail.  Some 300 fliers prefer no more than flaps 30&n= bsp;on=20 landing.
 
On the other hand, if rear CG is a concern in cruise flight, ticking= the=20 flap a bit out of reflex may help resolve this when in such an uncomfortab= le=20 flight regime.  A slightly slower speed must be endured and if the fl= ap is=20 taken completely out of reflex (normal wing), the IAS should probably be= no=20 greater than Va because of the stress on the wing/flap combo.  Note= that=20 Lancair's chief pilot recommended that the 200 series flaps be fully= =20 reflexed above 140 KIAS (or was it 160?).  All 300 series with standa= rd=20 landing gear should be using the Lancair recommended 5/16" (?,=20 >1/4") final push rods since the 1/4" ones were known to be= capable=20 of being bent.  Outback gear uses a different deployment technique.
 
I am not sure a bigger tail helps when one approaches the "neutral=20 point."  I once flew in a rear CG heavy condition where even the auto= pilot=20 couldn't quite hold level flight - instead a mild sinusoidal line was flow= n=20 until 60 pounds of fuel was burned off.  Too bad I didn't try to remo= ve=20 some reflex from the flaps - I was younger then.
 
It would be interesting to know if a test pilot did stalls or other= unusual=20 attitudes with the flaps in different positions.  Since some LNC= 2s=20 have fallen out of the sky at slow speeds and close to the ground, perhaps= it=20 would have been useful to know the flap position.  Because of the cri= tical=20 positioning of the partial rib where the final flap bell cranks are attach= ed,=20 the rate/angle of each flap could be different as it is moved through its= =20 range.  Indeed, flap rigging has often been used to eliminate a heavy= wing=20 at cruise - ergo, deployment of each could be a bit different.
 
It will be interesting if you are able to compute or formulate the=20 computation for the CG range limits depending on the wing shape.
 
Scott Krueger
320, small tail, std gear, gap seals
 
In a message dated 7/10/2010 8:28:50 P.M. Central Daylight Time,=20 Wolfgang@MiCom.net writes:
The LNC2 uses the NLF(1)-0215F airfoil. A lot can be found by doing= a=20 Google search on that number.
More detail can be found by going to Google for "NASA Technica= l=20 Paper 1865".
 
I have not taken the time to reverse engineer the CG range of the= LNC2=20 but let me offer some observations.
 
The airfoil used has long been touted as "the greatest thing since= sliced=20 bread" for General Aviation and it definitely has some advantages. But= it's=20 not new. Compare this airfoil to the P-51 airfoil and you will see some= close=20 similarities. The LNC2 being composite construction instead of aluminum= lets=20 the airfoil show more of it's theoretical advantages.
 
It's a laminar shape with a good drag bucket. That bucket can be ma= de to=20 move to the lower Cl (lift coefficient) ranges with reflex allowing noti= ceably=20 lower drag at higher cruise speeds. Along with reflex, the Cm (moment=20 coefficient) goes positive, the center of lift of the wing travels forwa= rd=20 giving a nose up force requiring down trim. This is in addition to the= usual=20 nose up force that goes with most all airfoils at high speed before= =20 considering flaps.
 
With down flap, the drag bucket will move to higher Cl's making slo= wer=20 flight more efficient. And, of course, the Cm goes negative giving a nos= e down=20 force requiring up trim.
 
. . . and appropriate variations in-between . . .
 
So, the rear CG limit is determined by high speed flight and availa= ble=20 control authority,
and the forward CG is determined by low speed / landing flight and= =20 available control authority.
 
What is becoming clear here is that the center of lift does quite= a bit=20 of traveling fore and aft which is exaggerated by allowing negative or= =20 "cruise" flaps. Since you can't shift the CG during flight, you need a= large=20 amount of pitch authority from the tail to cover that range of lift=20 travel.
 
You have two choices in the LNC2, live with the limitations or inst= all a=20 larger tail to give that extra pitch authority.
. . . A larger tail area can also help with abnormal=20 attitude recovery.
 
Wolfgang
 
From: Terrence O'Neill=20 <troneill@charter.net>
Subject: Re: [LML] LNC2 CG range Query<= /TD>
Date: Wed, 07 Jul 2010 09:32:19=20 -0400
To: lml@lancaironli= ne.net
Dear humble sevant... or is it 'savant'?=20

See, we often shackle our efforts to understand by having afo= rehand=20 made up conventions to better understand something else.  I= feel=20 motivared to offer some observations on understanding the L2s, if= I=20 may.
Don't forget that you have a delete button if this gets too= =20 boring.
This is how I see the Lancair's wing-flap-stability-stall, as= =20 compared to recent LML discussions of same.
The CG range and the wing's characteristics is a good example= of=20 thinking 'in the box'..  Guys working in the wind tunnels mea= suring=20 forces of invisible air found that at those small angles where win= gs=20 make a lot more z force than Y force -- those forces seemed to be= =20 centered around 25% chord... which they first called a center of= =20 pressure, and then 'aerodynamic center' or a.c., and so they made= their=20 charts about this imaginary point and things work out great for=20 calculations.
The thing is, what's really going on is that as the moving wi= ng=20 flows through the stationary air, a pressure bubble is generated,= with=20 most of it being at the leading edge, and then tapering=20 'triangular-sish-ly' toward the trailing edge.... at least at high= er=20 angles of attack.  And the lower or cruise angle so attack th= e=20 bubble flattens out aft-ward... and is varied by moving a trailing= edge=20 flap.
So this is getting at the importance of the bubble... that's= what=20 we move around when we move the pitch control.  With a flap,= or an=20 aileron, or  a tail flap thing.  
And so, when we talk about flying with the CG range forward= or aft,=20 we're really talking about the CG moving with respect to this=20 total-airplane-bubble... which we have to keep centered over our= center=20 of mass.  We do this by making little bubbles of pressure on= the=20 tail, or in Lancair reflexing of the flap, by moving the bubble of= the=20 wing.  And that's what the CG or center of mass is hanging=20 from.
So when we drop a flap, it's obvious that we have moved that= big=20 bubble, and have to balance it with changing the bubble size on th= e=20 horizontal tail.,, trim tabbing it.
The design characteristics of the L@'s reflexible airfoil are= =20 referenced to the section with the flap not reflexed.  Also,= =20 therefore, it is most likely that Lance figured the CG that way,= because=20 that's the way the aerodynamic data was available to him in the NA= SA=20 report, I think, on the NLF(1)-0215F... please hasten to correct= me if=20 I'm wrong. : )
I just try to keep my (total airplane) bubble's center as clo= se to=20 the CG, wherever that is, by making a little bubble over or under= the=20 horizontal tail, with the pitch trim tab.
One problem is that we pilots don;t talk much abut the shape= or=20 location of the pressure bubble at AOAs higher than the stall= =20 angle... and that's a situation where designers then have to= start=20 gluing yukky shapes of strakes and vanes on to correct this oversi= ght.=20  What Lance should have done in the first place is wind-tunne= l his=20 200 and -235 from zero AOA up to 90 degrees, and he would have see= n a=20 big forward movement of the bubble's center resulting from the bro= ad=20 cowl and skinny aft fuselage... imo.
Such testing was done by NASA for the Piper canard (after it= =20 crashed) and on a configuration like the Dragonfly... which I disc= overed=20 when researching the Dragonfly we bought... and discussed in a Kit= planes=20 article many years ago.  In the too frequent event that a new= =20  design configuration is locked in by building before testing= , the=20 economical remedy solution then is to fly with a forward CG, or to= add=20 strakes aft, to keep that total-airplane high AOA bubble centered= where=20 it belongs ... aft of the design CG,.. for a restoring pitching=20 moment
In the Dragonfly we flew at forward CG.
In L235/320 N211AL I have modified the horizontal tail to add= slots=20 to prevent  it from stalling at AOA higher than the wing's st= all=20 AOA...( still testing that, but it worked on my Magnum.)  Als= o I=20 added ballast to the engine mounts to be certain the CG stayed for= ward,=20 within Lance's original limits.  
It's a beautiful little plane, and very efficient ... and thi= s is=20 how it works -- I think.

Terrence
L235/320 N211AL



On Jul 7, 2010, at 2:18 AM, Sky2high@aol.c= om=20 wrote:

If you know or even care:
 
In general, LNC2's as originally designed seemed to better= =20 tolerate a CG at the forward edge of the envelope rather than fl= ight=20 at or towards the rear.  This includes adequate elevator co= ntrol=20 at flare during landing.  Lancair tested the long engine mo= unt on=20 an LNC2 that moved the forward CG edge +1.5" and there were= no=20 flight problems.  Hmmmmmm.........
 
Consider that the LNC2 wing has a dramatic change in= pitch=20 forces when the flap is moved between its designed standard posi= tion=20 and into -7 degrees reflex.  In my airplane at around 140-1= 60=20 KIAS the difference in those two flap positions=20 is approximately a measured 6 degrees in attitude (cou= ldn't=20 measure the AOA delta).  It is clear that moving the flaps= =20 sightly out of reflex (1 or 2 degrees) can help resolve uncomfor= table=20 flight at rear CG conditions by pitching the nose down some and= =20 altering the AOA.  Perhaps the rear CG and small tail at cr= uise=20 leads to some flight instability that cannot be overcome by the= size=20 of the tail? 
 
So, here is the question:  If the CG range was calcula= ted=20 for the normal state of the wing (flaps not in reflex), is it po= ssible=20 that the range is too far back for normal cruise flight with the= flaps=20 in full reflex?  If so, should the POH aircraft data= include=20 two ranges based on these two flap positions?  What do= es=20 such a change do to the forward CG limit?
 
Of course, this might raise the same question with the 200= series=20 aircraft.  Why?  Well, the faired in position of the= flaps=20 for 200 series aircraft is the not-in-reflex position while the= plane=20 cruises with the flaps reflexed and not faired in.  The 300= =20 series aircraft has the flaps in reflex when they are faired in= to the=20 fuselage.     
 
When considering an answer, remember that wings designed to= =20 operate by changing shape (TE goes through some reflex angles)= have=20 been primarily used in tailless airplanes and the TE=20 position controls the pitch balance of the airplane. = I have=20 no idea how the CG range for such an aircraft is determined.
 
Your humble servant,
 
Grayhawk
 
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